Bugzilla is the leading open-source/free software bug tracking system, with high-profile installations at mozilla.org, GNOME, Red Hat, and NASA, among others. It features a comprehensive set of fields, attachment management, inter-bug dependencies, email notification of changes, a powerful query interface, reporting, and Web, XML, email, and console interfaces. It can be managed entirely using a Web-based UI. The user UI is customizable using templates.

Mantis Bug Tracker (MantisBT) is a Web-based issue tracking system that is both simple to use and highly customisable. A plugin system allows users to easily create and add new features. MantisBT is over 10 years old and is actively developed.

RT is an industrial-grade trouble ticketing system. It lets a group of people intelligently and efficiently manage requests submitted by a community of users. RT is used by systems administrators, customer support staffs, NOCs, developers, and even marketing departments to track issues, outages, bugs, requests, and all kinds of other things at thousands of sites around the world.

TWiki is a flexible, powerful, and simple Web based collaboration platform. It is suitable for dynamic intranets and knowledge bases, and for sharing and managing documents and collaborative projects. It resembles a normal Web site, but every page can be changed from a browser. It features automatic link generation, full text search, group authorization, Web forms, reporting, change notification, file attachments, revision control of pages and attachments, a modular templating system with skins, hierarchical navigation based on the topic parenting feature, and more. Plugins can be used to enhance the program and build groupware applications.

The Apache Open For Business Project is an enterprise automation software project that includes ERP, CRM, E-Business/E-Commerce, SCM, MRP, and CMMS/EAM functions. It is a foundation and starting point for enterprise solutions and can certainly be used out of the box, but is also great for creating specialized applications.

PHP Trouble Ticket provides a centralized means of problem reporting
and tracking in multi-user environments (medium to large companies,
development groups, etc.). It helps direct problem reports to the
responsible administrator(s), keeps track of the status of a problem
being worked on, and offers an up-to-date list of recent changes
applied. It aims to be widely configurable, easy-to-use even for those
not technically inclined, and multi-language capable.

Ozibug is a platform-independent, Web-based bug tracking system that can be installed and run straight out of the box on any Java servlet capable platform. It can be used to manage the reporting of bugs and tracking of their progress from discovery through to resolution. It provides standard features such as file attachments, multiple module support, email notifications, custom reports, and RSS feeds. For those who require more flexibility, it also offers fine grained access control, database storage, pluggable authentication, an incoming mail gateway, and customizable appearance, email notifications, and reference data.

BATTS (Barnhard Associates Trouble Ticketing
System) is a lean, elegant trouble ticket system with both command line and Web interfaces, written in Perl. It uses MySQL for its backend, and has a ticket-via-email interface for submission and informational logging. Features include support for tickets pending on or resolved when other tickets are disposed of, an ability to assign tickets to a person, an ability to categorize and prioritize tickets, and associating billing codes with various logging events.

Bugzero is a Web-based change management and issue tracking system used in a distributed team environment to track software bugs, hardware defects, test cases, or any other issues. It can also be used equally well as a helpdesk customer support, trouble ticketing, or email management system to collect and manage customer feedbacks, incidents, requests, and issues. It is easy to use, but still flexible and adaptive, and can be configured to fit to your organization's unique business process and workflow.